In a blast furnace producing pig iron, iron ore is charged together with coke and fluxes through the upper part of the furnace. A hot air blast is injected through tuyeres at the bottom of the furnace, thereby generating heat by the combustion of the carbon of coke which melts down the charge. The controlled combustion of coke also generates hydrogen and carbon monoxide which chemically reduce the iron oxides in the furnace. Periodically liquid iron and slag are tapped from the furnace. The combustion gases flow up through the furnace and reduce the iron oxides, and exit the furnace as a stream of dust-laden hot gas. Heat is recovered for preheating the air blast, and this furnace top gas, once cooled down, is then normally used as fuel in other areas of the steel plant.
Metallurgical coke is needed in the charge of a blast furnace because this material (produced by pyrolysis, e.g. indirect heating without oxygen presence, of coal in coke ovens) provides the structural support of the charge of the furnace above the so-called “dead man” zone where the metallic iron starts melting and falling down to the bottom part of the furnace where molten iron and slag are collected.
Coke also provides the heat for melting the iron charge by its combustion with an oxygen-containing gas, typically preheated air, the combustion gases, mainly composed of CO and CO2 with some H2 and water, flow upwardly through the shaft portion of the furnace and reduce the iron oxides to wustite (FeO).
Several proposals for recycling top gas in a blast furnace with the aim of reducing the coke rate, are found in the prior art, addressed to recycling top gas to the furnace and in this way decrease the coke consumption to a minimum. If the top gas is heated in a direct fired tubular heater, the tubes of the heater made of high-grade alloys to withstand the high temperatures necessary to raise the temperature of the top gas above, 850° C., preferably in the range of 900° C. to 1100° C., need to follow a specific scheduled temperature profile during start-up and shut-down periods, to avoid thermal shock to the tubes, which requires a continuous flow of the gas through the tubes of the heater during such start-up and shut-down.
Applicants have found several patents and patent applications concerning top gas recycle to a blast furnace for reducing coke consumption, teaching that the recycled gas is to be heated to temperatures suitable for direct reduction of iron oxides, for example: U.S. Pat. Nos. 3,784,370; 4,844,737; 4,917,727; 4,363,654; 5,234,490; U.S. Patent application No. 2010/0212457 A1; British Patent No. GB1,218,912; and Japanese Patent Publication No. JP55113814.
None of the above patents or patent applications teach or suggest providing an alternative path comprising cooling means for the hot gas effluent from the gas heater for protecting said gas heater components from thermal shocks due to unforeseen operation interruptions of the blast furnace. Solutions for this practical problem of the heaters for improving the operation and availability of blast furnaces with upgraded top gas recycle are not envisioned in the prior art.